Frozen in Time: What Caused the Extinction of the Ice Age Megafauna?

A new study of ancient DNA preserved in permafrost has revealed that Ice Age megafauna varied considerably in their ability to survive climate change and the spread of humans.

Over the past 50,000 years the world has lost many of its large animals. Among the “megafauna” species that disappeared were iconic mammals like the woolly mammoth and woolly rhinoceros, which roamed the tundra during the last Ice Age. The intensity and timing of these megafauna extinctions varied among different regions of the world.

The reasons for these extinctions have long been a source of interest to researchers and the general public alike. Scientists in the early 19th century, including Georges Cuvier, William Buckland and Louis Agassiz, believed that the megafauna were wiped out by catastrophic geological events such as floods.

But after it was demonstrated that humans actually co-existed with some of the species of megafauna, there was a growing suspicion that overhunting played a role in the extinctions. Such noted biologists as Richard Owen and Alfred Russel Wallace placed the blame on humans.

Now the debate has seen most researchers falling into one of two camps, with the argument revolving around the impacts of climate change and humans. Did the megafauna succumb to the effects of a changing climate? Or did humans drive them to extinction by hunting them, modifying their habitat, or introducing diseases?

It is also important to consider why other large animals, such as those that we see today in the African fauna, managed to withstand these pressures.

Ever since modern humans first dispersed out of Africa, megafauna extinctions have roughly coincided with the movements of humans across the globe. In some places, especially islands, the extinctions occurred merely hundreds of years ago. Well-known examples are the moa of New Zealand and the elephant birds of Madagascar. There is overwhelming evidence that we hunted these large flightless birds to extinction.

In other parts of the world, such as Europe and Australia, the megafauna disappeared tens of thousands of years ago. Although it is possible that they suffered at the hands of humans, there were also major climatic events throughout this period. A changing climate would have greatly altered the distribution of suitable habitats.

Because most of the megafauna extinctions happened so long ago, the traces that they have left in the fossil and archaeological records are rather fragmentary. Different researchers have had conflicting interpretations of this evidence, which is one of the main reasons why there is ongoing debate over the causes of the extinctions.

Studies of the megafauna extinctions strongly depend on estimates of dates. We can try to date the disappearance of a species by estimating the ages of preserved bones and other organic material. Then we can use archaeological evidence to estimate the timing of human arrival and occupation.

Estimating specimen ages is often done directly by radiocarbon dating, which can be used on organic material such as bones and hair. Other methods are available for dating surrounding sediments and inorganic artefacts. The various dating methods are effective over different timescales and have different degrees of reliability and precision.

Based on the date estimates, we can work out for how long humans and megafauna lived alongside one another, if at all. If the megafauna disappeared soon after humans arrived it suggests that humans played a role in the extinctions.

Sometimes we have no evidence that humans actually hunted these species, or that humans had the capacity to drive the animals to extinction. We can try to answer these questions by seeing whether megafauna remains can be found in archaeological sites.

In recent years, huge progress has been made in methods for genetic analysis. It is now possible to study the populations of extinct animals using DNA from ancient specimens. For example, we can extract DNA from preserved bones, hair, feathers and even eggshells. This can only be done in specialised laboratories because the DNA molecules tend to be highly degraded.

The DNA from these ancient specimens contains information about population history. For example, if the DNA varies greatly between individuals from the same species it suggests that the species had a large population. Using the appropriate statistical methods we can analyse the DNA to study ancient populations and to estimate how they changed over time.

Climate, Humans or Neither?
In a recent study published in Nature, my colleagues and I used a range of techniques to study megafauna extinctions in the Northern Hemisphere. We focused on six species: woolly mammoth, woolly rhinoceros, steppe bison, muskoxen, wild horse and reindeer. For all of these species, the permafrost has preserved large numbers of specimens that we were able to analyse using DNA techniques.

The study involved the combined efforts of a large team of geneticists, archaeologists and zoologists. We put together the largest data set of its kind, including nearly 10,000 radiocarbon dates along with DNA sequences from more than 800 specimens. Collecting these samples and extracting their DNA took years of work.

For each of the six megafauna species we considered the effects of climate change and humans in turn. With such a large data set at our disposal, we were able to analyse these two factors in considerable detail.

First, we studied the effects of climate change by examining how the megafauna populations responded to shifting habitats and significant climatic events. To do this, we began by analysing the DNA sequences to estimate the past population sizes of the six species.

We found that the population sizes of the six species were closely related to the amount of available habitat, which is determined by climatic factors. We also found that the timing of large changes in population size corresponded to major climatic events, such as the peak of the last ice age around 22,000 years ago. These lines of evidence strongly suggest that the climate has been a major driver of the megafauna populations that we studied.

In the second part of our study we used a number of methods to estimate the impact of humans on the six megafauna species. We studied the distribution of archaeological sites to determine whether humans and the six animal species were present in the same places at the same time. The presence or absence of megafauna remains at these sites also hinted at the degree of human interaction with these species.

For the woolly rhinoceros and the Eurasian woolly mammoth, populations actually increased after their first contact with humans. For the muskoxen we found that there was little geographic overlap with humans. Muskoxen remains were rarely found at archaeological sites, suggesting a limited amount of interaction. Therefore humans probably did not much have much of an overall impact on the populations of these three megafauna species.

While the declining populations of woolly rhinoceros and muskoxen could be explained by changes in climate without any human intervention, we could not find a clear cause for the extinction of the woolly mammoth. In fact, a dwarfed version of the woolly mammoth managed to survive on Wrangel Island, off the north-eastern coast of Siberia, until it finally became extinct only 3700 years ago.

We found evidence of human impact on populations of wild horses and bison. Both species suffered population crashes upon contact with humans and are the most common species found in the archaeological sites that we studied. Wild horses were found at about 60% of the European and Siberian sites, whereas bison were found at nearly 80% of the Siberian sites.

The reindeer also suffered upon contact with humans, but has managed to thrive to the present day. This might be because the reindeer has a higher rate of reproduction and is ecologically flexible. Furthermore, populations of reindeer have remained quite well-connected to each other over time.

Our study shows that different species of megafauna can respond very differently to human impacts and changing climate. The causes of extinction can be complex and cannot always explained by a single factor. This makes it difficult to predict the future of modern populations of animals, which face pressures from humans, habitat loss and rapid climate change.

Extinctions in Australia
Australia has lost an assortment of wondrous megafauna species. In the late Pleistocene our landscape would have been dominated by impressive marsupials such as the diprotodon, which was a hippo-sized animal closely related to the wombat. There were also a number of large flightless birds and giant versions of many modern marsupials and monotremes.

The study of megafauna extinctions in Australia has been very challenging. In particular, it has not been easy to obtain reliable dates for the extinctions. Unfortunately, the arrival of humans and many of the extinction events happened around 50,000 years ago, which is at the upper limit of radiocarbon dating. Other dating methods are now being used to get a better idea of the timeframe of extinction.

Owing to the harsh climatic conditions in Australia, it is unlikely that we will ever be able to conduct a comprehensive genetic study on Australia’s megafauna extinctions. DNA molecules quickly degrade in the heat and are very unlikely to survive even in the most well-preserved specimens. Our best hope is to look for specimens that have been deposited in cold, dark caves.

It seems likely that the debate about the megafauna extinctions in Australasia will persist for some time. The continuing level of disagreement indicates that much work remains to be done. We can only hope that studies of DNA hold the key to solving one of the greatest mysteries of Australia’s pre­history.

Simon Ho is a Senior Lecturer and an Australian Research Council Queen Elizabeth II Research Fellow in the School of Biological Sciences at the University of Sydney.